1. Field of the Invention
The present invention relates to a sensitivity correction method and an image reading device, and in particular to a sensitivity correction method and an image reading device for eliminating the influence of shading from a read image when an original is irradiated with light, the light transmitted therethrough or reflected thereby is detected by photoelectric conversion elements, and an image recorded on the original is read.
2. Description of the Related Art
In recent years, an image reading device has been put to practical use which, while a photographic film is conveyed in a sub-scan direction at a constant reading speed, reads a film image recorded on the photographic film by a three-linear CCD (Charge Coupled Device) which converts an amount of received light at each channel corresponding to the respective each colors of RGB into color signals, performs processings such as various corrections on image data obtained by this reading, and then, carries out image recording onto a recording material such as photographic printing paper, or carries out image display on a display, or the like. Such an image reading device has the merit that, as compared with a conventional image reading device in which a film image is recorded on photographic printing paper by surface exposure, it is easy to automate the operations from reading of an image recorded on a photographic film to recording of an image onto a recording material such as photographic printing paper.
Here, in the image reading device described above, the photographic film is irradiated with light and transmitted light is detected so that the film image is read (so-called transmission type image reading device), or light reflected from the photographic film is detected so that the film image is read (so-called reflection type image reading device). At this time, in the linear CCD, wavelength characteristics of light detection sensitivity of the CCD fluctuate for each pixel (see FIG. 8A), and it is difficult to uniformly irradiate the photographic film with light from a light source and thus amount of irradiation light is irregular (see FIG. 8B). Shading is thereby caused on a read image (see FIG. 8C). In the image reading device, sensitivity correction is carried out for each pixel to prevent an influence of the shading from being exhibited in the read image.
Specifically, for example, before the start of image reading, in the transmission type image forming apparatus, a state (blank state) where a photographic film is not set is read by a CCD, shading caused by differences in light sources and differences in sensitivities of each pixel of the CCD is detected, and a correction amount is obtained in advance for each pixel so that the results of reading (CCD output) become uniform and the respective RGB colors are balanced with each other. At the time of reading the film image, this correction amount is used and correction is carried out on the CCD output obtained at the time of reading the film image, so that the influence of the shading is eliminated from the read image. In the reflection type image forming apparatus, a white reference plate is set instead of a photographic film, and a correction amount is obtained from the results of reading (CCD output).
It is known that the spectral characteristic (spectral transmission characteristic/spectral reflection characteristic) of a photographic film differs in accordance with the type of the film, such as negative, positive, DCN, black-and-white, sepia, or the like. For example, in the case of a positive film, the transmissivity (or reflectivity) is substantially constant regardless of the wavelength of light (wavelengths in the visible region), whereas in the case of a negative film, because the base portion is colored, the transmissivity (or reflectivity) is dependent upon wavelength.
Thus, in the case where an image recorded on a negative film is read, the light amount balance at the time of reading deviate from the light amount balance at the time when the correction amount is obtained. Thus, a satisfactory shading eliminating effect cannot be obtained by the sensitivity correction of conventional techniques, and a high quality read image cannot be obtained.